As a flow rate measurement method, various methods have already been proposed. However, in a conventional flow rate measurement method, there has been a problem that attenuation or a phase lag often occurs due to the characteristic of the measurement method itself, so that the pulsatingly fluctuating flow rate cannot be measured except under a limited condition (FIG. 1). Some conventional flow rate measurement methods will be overviewed below.
The Non-Patent Document 1 discloses an example of measuring a pulsatingly fluctuating flow up to about 50 Hz using an electromagnetic type flow rate measurement apparatus. It is believed that the electromagnetic type exhibits lower attenuation or phase lag at a relatively low frequency because it does not have any mechanical moving part. However, applicability of this type is limited, due to its principle, to a fluid having electrical conductivity (typically, water).
The Non-Patent Documents 2 and 3 disclose an example of measuring a pulsatingly fluctuating flow of liquid nitrogen using an ultrasonic type flow rate measurement apparatus. The ultrasonic type is applicable to any fluid as long as the ultrasonic wave is propagated therethrough. Further, the ultrasonic type exhibits lower attenuation or phase lag in principle because it does not have any mechanical moving part. However, this type is required to perform a high-load calculation to convert the ultrasonic signal into a flow rate value, so that, when the calculation is sequentially performed to output a flow rate signal during measurement, a phase lag corresponding to the calculation time occurs. For example, as illustrated in FIG. 2, when an actual flow rate varies over time (the horizontal axis represents a time) as indicated by (1), a graph representing the time variation of the flow rate can be provided as indicated by (5) according to the processes of: transmitting an ultrasonic wave into the fluid at each timing separated by appropriate time intervals as indicated by (2), while receiving the ultrasonic wave propagated within the fluid; sequentially calculating the obtained ultrasonic signal to convert into flow rate values as indicated by (3); and recording the flow rate signal upon completion of the conversion as indicated by (4). However, the recorded flow rate signal includes a time lag corresponding to the calculation time. To reduce the time lag, it is required to employ a simple and low-load calculation method. However, such a calculation method has a problem that it is susceptible to a noise and is not necessarily capable of correctly achieve the conversion into the flow rate value.
The Non-Patent Documents 4 and 5 disclose an example of determining a flow rate from a pressure difference between two points in a flow passage. Generally, the pressure measurement exhibits lower attenuation or time lag as compared to the flow rate measurement. However, to determine the flow rate from the pressure difference, it is necessary to know flow passage resistance and fluid inertia, but it is difficult to measure these values directly. Thus, this method has a problem that a theoretical value etc. must be assumed.
The Patent Documents 1 and 2 disclose a technique of measuring a flow rate at one point at a time for each cycle of pulsating fluctuation by taking as an example intake and exhaust of an automobile engine.